Truth serum is a drug used to obtain information during an investigation from subjects who are unwilling to answer or somehow unable to remember the facts first-hand. There are several drugs that can be used in this way, including sodium pentothal, sodium thiopental (an anesthetic), grain alcohol (ethanol), scopolamine (a highly toxic depressant), and barbiturates. The same drugs have many other applications: they are used in psychiatry to treat phobias, as a general anesthesia, and even to produce medically-induced comas.
The truth serum has often been portrayed in fiction and films as a magical solution. Even the character Barty Crouch in the "Harry Potter" books is subjected to a magical version and ends up confessing to a list of offenses. Many films, especially those that came out a couple of decades ago, when the use of such drugs was at its peak of popularity, portrayed the serum as 100% effective. TV series 24, docudrama Shell Shock, and many others have used it as a plot device.
In real life, however, the use of these drugs as a truth serum is highly controversial. While they typically do make a person more likely to tell the truth, they also make him more likely to get confused about what he is saying. As the person becomes more and more talkative, the lines between fact and fantasy begin to blur. Experts believe that up to 50% of what a person says while under the influence of such drugs is either an embellished version of the truth or a complete invention. It produces an effect similar to alcohol intoxication, lowering inhibitions, and making people chattier and more prone to answering questions.
Military intelligence in both the US and Russia are testing other drugs as a potential replacement for the ones currently in use. Both countries still use these medications in controversial cases where other methods have failed to provide an answer, including high-profile military cases and circumstances involving national security matters.